1. Field of the Invention
The present invention relates to a photometric device for improving performance by removing each kind of noises.
2. Description of the Prior Art
There is utilized a two-wavelength photometry for lessening an influence of absorption, scattering or the like caused by scratches or stains of a detector such as a photometric cell or foreign matters or the like in a solution to be detected in quantitative analysis by a colorimetric method. The two-wavelength photometry uses two different kinds of wavelengths such as a maximum sample absorption wavelength .lambda..sub.1 and a less sample absorption wavelength .lambda..sub.2, and as a light absorption amount of an actual sample, a light absorption amount in case of the wavelength .lambda..sub.2 is substracted from a light absorption amount in case of the wavelength .lambda..sub.1, a difference between both is sought and a light absorption amount in the wavelength .lambda..sub.1 of the actual sample is obtained.
FIG. 1a shows one embodiment of an output waveform in case of receiving two lights of a wavelength .lambda..sub.1 and a wavelength .lambda..sub.2 by a single detector in a photometric device by such two wavelength method, wherein the portion of the wavelength .lambda..sub.1 and the portion of the wavelength .lambda..sub.2 alternately appear at suitable intervals.
With the use of a wavelength discriminating signal shown in FIG. 1b, discrimination of the wavelength is carried out by a circuit having a construction shown in FIG. 2. In FIG. 2, reference numeral 1 is a detector, reference numeral 2 is a discriminating signal generating circuit, and a switch 3 is switched by an output signal of the circuit 2 so as to obtain two outputs. That is, in case of closing a contact 3a of the switch 3, the output for the wavelength .lambda..sub.1 shown in FIG. 1c is obtained, while in case of closing a contact 3b of the switch 3, the output for the wavelength .lambda..sub.2 in FIG. 1d is obtained.
These signals contain a shadowed portion which does not vary by the amplitude of a photometric signal. When this shadowed portion is existent, even if the amplitudes of the photometric signals for the wavelengths .lambda..sub.1 and .lambda..sub.2 are varied at the same ratio, a ratio .lambda..sub.1 /.lambda..sub.2 thereof cannot be kept constantly and as a result, linearity of characteristic as a whole of the photometric device is deteriorated.
The cause of the shadowed portion is due to stray light of the optical system or dark current of the detector. There is also a method of removing the shadowed portion with the use of a preamplifier instead of direct wavelength discrimination of the detector output, but in this case, a temperature drift of an amplifier circuit or a shift of offset adjustment becomes a problem. Further, the dark current of the detector and the stray light are influenced by the surroundings and are not always certain. There is further a method of discriminating a wavelength by sampling and holding a point X and a point Y of FIG. 1a, but in this case, the above-described disadvantage cannot be avoided.